I got my wheelchair first and then started looking for a van. Like you, I discovered the price of new conversion vans and said "no way".
I also discovered that very few vans could accommodate my wheelchair. I was able to identify a few vans that would work and started my search. I used many online resources and finally found my van on craigslist in a city about 3 hours away.
Overall, it took about 2 months of serious searching to find the van. That van is a compromise from what I really wanted, but it has worked out well.
I am an optimist, and consider this my starter van
I hope to replace it with another chair hauler someday that satisfies some of my more unusual desires (4 wheel driver, high ground clearance, able to tow a trailer).
I found that in my area, wheelchair vans go very quickly. I stacked the deck in my favor by having cash ready and searching daily. When I found the fan on craigslist, I moved on it instantly. Had a waited a day or two, it would have certainly been gone.
Good luck, and post pictures of what you wind up with!
Steve